This book is for you if you want to learn the inner workings of computer graphics and do all the stuff the cool kids do by yourself. Doing things by yourself is extremely fun and resourceful and gives you a great understanding of graphics programming.
OpenGL is mainly considered an API (an Application Programming Interface) that provides us with a large set of functions that we can use to manipulate graphics and images. However, OpenGL by itself is not an API, but merely a specification, developed and maintained by the Khronos Group.
Learn OpenGL is broken down into a number of general subjects. Each subject contains several sections that each explain different concepts in large detail. Each of the subjects can be found at the menu to your left. The subjects are taught in a linear fashion (so it is advised to start from the top to the bottom, unless otherwise instructed) where each page explains the background theory and the practical aspects.